Beta-keratins of the crocodilian epidermis: composition, structure, and phylogenetic relationships.
Nucleotide and deduced amino acid sequences of three beta-keratins of Nile crocodile scales are presented. Using 5'- and 3'-RACE analysis, two cDNA sequences of 1 kb (Cr-gptrp-1) and 1.5 kb (Cr-gptrp-2) were determined, corresponding to 17.4 and 19.3 kDa proteins, respectively, and a pI of 8.0. In genomic DNA amplifications, we determined that the 5'-UTR of Cr-gptrp-2 contains an intron of 621 nucleotides. In addition, we isolated a third gene (Cr-gptrp-3) in genomic DNA amplifications that exhibits seven amino acid differences with Cr-gptrp-2. Genomic organization of the sequenced crocodilian beta-keratin genes is similar to avian beta-keratin genes. Deduced proteins are rich in glycine, proline, serine, and tyrosine, and contain cysteines toward the N- and C-terminal regions, likely for the formation of disulfide bonds. Prediction of the secondary structure suggests that the central core box of 20 amino acids contains two beta-strands and has 75-90% identity with chick beta-keratins. Toward the C-terminus, numerous glycine-glycine-tyrosine and glycine-glycine-leucine repeats are present, which may contribute to making crocodile scales hard. In situ hybridization shows expression of beta-keratin genes in differentiating beta-cells of epidermal transitional layers. Phylogenetic analysis of the available archosaurian and lepidosaurian beta-keratins suggests that feather keratins diversified early from nonfeather keratins, deep in archosaur evolution. However, only the complete knowledge of all crocodilian beta-keratins will confirm whether feather keratins have an origin independent of those in bird scales, which preceded the split between birds and crocodiles.
Dalla Valle
Luisa
L
Dipartimento di Biologia, Università di Padova, Padova, Italy.
Nardi
Alessia
A
Gelmi
Carlotta
C
Toni
Mattia
M
Emera
Deena
D
Alibardi
Lorenzo
L
D000489:Alligators and Crocodiles / Q000235:genetics / Q000378:metabolism*; D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D001499:Bayes Theorem; D015139:Blotting, Southern; D003001:Cloning, Molecular; D017931:DNA Primers / Q000235:genetics; D018076:DNA, Complementary / Q000235:genetics; D004817:Epidermis / Q000033:anatomy & histology / Q000378:metabolism*; D007150:Immunohistochemistry; D017403:In Situ Hybridization; D016013:Likelihood Functions; D008957:Models, Genetic; D008969:Molecular Sequence Data; D010802:Phylogeny; D011487:Protein Conformation; D017422:Sequence Analysis, DNA; D053800:beta-Keratins / Q000235:genetics / Q000378:metabolism*
D017931:DNA Primers; D018076:DNA, Complementary; D053800:beta-Keratins